In analysis of a sample by mass spectrometry it is known to release molecules from the solid surface of a sample by the interaction of a sudden, intense beam of laser radiation. With this technique a significant number of such molecules are released as ions and it is known to collect and focus such ions into a time of flight mass spectrometer to facilitate the analysis of the surface and near surface regions of the sample. This technique is known as Laser Desorption Ionization Mass Spectrometry (LDI MS).
When a sample is composed principally of a low molecular weight aromatic matrix within which are dispersed trace quantities of analyte molecules of peptides and proteins then bio-molecules can be released from the sample. The mechanism for this release stems from the nature of the matrix molecules. Such molecules are chosen to contain a chromaphore structure which absorbs strongly at the wave length of the irradiating laser. Usually, this wavelength is within the ultra-violet part of the spectrum. When the laser radiation exceeds a threshold level of ˜100-200 J/m2 the matrix suffers a violent surface desorption as matrix material and included analyte molecules are released into vacuum. Ionization of the molecules is thought to occur subsequently by proton attachment as a by-product of the fragmentation of the matrix molecules which are rich in protons. The matrix in this ‘matrix assisted’ LDI thus has two purposes: one to enhance the absorption of the ultra-violet radiation and two, to provide a rich source of protons. This technique is well known as MALDI MS. It is known that different analytes can be desorbed preferentially in different matrices and that the optimum, threshold laser irradiance can differ for different analyte/matrix combinations. It is, therefore, necessary to be able to vary the laser irradiance incident upon the sample and this is conventionally accomplished using some form of variable neutral density filter to achieve attenuation of the laser beam. A disadvantage of this method is that precious laser light which could be used for analysis of the sample is irretrievably lost in the attenuator. This extends the experimental timescales for this type of analysis.